Agreed, asking for the best default DE for the next version and asking for
your favorite setup is really not the same.
For weaker machines, there's Trisquel Mini. Or a minimal netinstall.
Accessibility must be there for both the default and Mini version I suppose.
From there, there's the 3D accel thing. Maybe there's a way to detect if it's
supported, and go on with the install accordingly.
My favorite setup ?
I like i3wm : very fast, light, and plain text configuration files (easy to
modify).
And Openbox the way Crunchbang sets it up is nice and fast for a more classic
appearance/use.